1. Field of the Invention
The present invention relates to a method of and apparatus for manufacturing and evaluating an optical fiber coupler, in which a plurality of polarization maintaining optical fibers are partially heated so that they are conjoined together through fusion and elongated to constitute the coupler.
2. Related Art
An optical fiber coupler is a device for branching or coupling light signals transmitting through a plurality of optical fibers. The coupler is formed by partially heating the plural optical fibers so that they are conjoined together through fusion and elongated to constitute the coupler.
The portion of the coupler, which is decreased in diameter because of the fusion conjoining and the elongation, is called a tapered portion, and is housed and secured in a casing.
An important characteristic parameter of the coupler is a crosstalk property indicative of the degree of the maintaining of the polarization of the light. The crosstalk CT in the coupler in the case of entering X-polarized light into the coupler is expressed by an equation (1) as follows: EQU CT=10.multidot.log (P.sub.Y /P.sub.X) (dB) (1)
where P.sub.X and P.sub.Y in the equation (1) denote the optical powers of the X-polarized and Y-polarized components of the light emitting out of the coupler, respectively. The crosstalk usually takes a negative value. The smaller the value is, the better the polarization maintaining property of the coupler is. To attain a good crosstalk property of the coupler, the polarization axes of the optical fibers need to be made coincident with each other during the manufacturing of the coupler.
FIG. 1 is a cutaway view showing a conventional procedure of making the polarization axes, of polarization maintaining optical fibers for such a coupler, coincide with each other. FIG. 1 shows an optical fibers 21 provided with a core part 22, a clad part 23 and a stress applying part 24, a microscope 25, an illumination light source 26, and rotary clamps 27. Before the optical fibers 21 are fused and conjoined together, light emitting from the light source 26 and transmitting through the fibers is observed through the microscope 25 to find out the directions of the stress applying parts 24, which determine the directions of the main polarization axes of the fibers. The rotary clamps 27 are then turned to rotate the optical fibers 21, and secured in such positions that the polarization axes of the fibers are coincident with each other.
However, the degree of the coincidence is not high enough in the conventional method. In other words, it is unavoidable to make some discrepancy between the polarization axes of the fibers. Further, even if the axes may perfectly coincide with each other before the fibers are fused and conjoined together, the fibers undergo a twist, or the like, during the fusion conjoining and/or elongation of the fibers to make the axes incoincident with each other to deteriorate the crosstalk property of the coupler at the time of the completion of the manufacturing thereof. This is a problem with the conventional method of manufacturing an optical fiber coupler.
On the other hand, since the coupler sometimes changes in properties, such as, branching ratio and excess loss property, depending on the state of polarization of light which enters into the coupler, the properties of which need to be evaluated as to polarization. If the coupler is constituted by polarization maintaining optical fibers, the crosstalk property of the coupler, which indicates how much the state of polarization of light entering into the coupler is preserved, is an important characteristic parameter. To conventionally evaluate the properties of such couplers as to polarization, measurement is performed on the coupler through the use of a measuring system as shown in FIG. 2.
For the above-mentioned conventional evaluation, light whose state of polarization is modulated by a polarizer 4, a wavelength plate 3 and so forth is required to be condensed by lenses 2 and then enters into the optical fiber 10 of the coupler 6 from one end thereof. To efficiently enter the light into the fiber 10, the light needs to be concentrated to the core of the fiber. For that purpose, a finely movable mechanical stage on which the fiber is put moves for adjustment so as to maximize the monitored power of the light emitting out of the fiber. It is necessary to set the fiber on the measuring system at each of both the ends of the coupler and move the mechanical stage for adjustment, at the time of each of the manufacturing and evaluation of the coupler. In case that the coupler is constituted by polarization maintaining optical fibers, it is necessary to not only perform the setting and the movement, but also make the polarization axis of the fiber coincide with that of the light which enters into the fiber. These are additional problems associated with the conventional methods of manufacturing and evaluating an optical fiber coupler because the conventional methods are quite intricate and take a lot of time for measurement steps.